This is a multicenter, open-label, Phase 1/2, dose-escalation and dose expansion study of a CXCR4 inhibitor, USL311, alone and in combination with lomustine in subjects with advanced solid tumors (Phase 1) and subjects with relapsed/recurrent GBM (Phase 2). The study is designed to explore the safety, tolerability, pharmacokinetics, and preliminary efficacy of USL311 alone and in combination with lomustine.